What Is Fog?
Fog is a collection of tiny water droplets, or in some very cold conditions ice crystals, suspended in the air near the Earth’s surface. When these particles become concentrated enough, they scatter light and reduce horizontal visibility.
Fog is common across Canada and can develop in many different environments, from coastal areas and lake shores to valleys, cities and the Arctic. It can form overnight when the ground cools, when warm moist air moves over a colder surface, or when additional moisture is introduced into cool air.
Although fog itself does not usually produce damaging weather, significant reductions in visibility can create hazardous conditions for drivers, aircraft and marine transportation.
Quick Facts About Fog
Main Characteristic: Reduced visibility near the Earth’s surface
Composition: Primarily tiny suspended water droplets
Formation: Air near the surface becomes saturated
Common Seasons: Possible year-round
Major Hazard: Reduced visibility
Common Locations: Coastal areas, valleys, lakeshores and low-lying terrain
Related Conditions: Dense fog, freezing fog, ice fog and mist
Related Alert: Fog Advisory
How Does Fog Form?
Fog generally forms when air close to the ground becomes sufficiently saturated for water vapour to condense into tiny droplets.
Air can become saturated in two main ways:
- The air cools until it reaches its dew point
- Additional moisture enters the air until saturation occurs
Once saturation is reached, water vapour can condense onto microscopic particles known as condensation nuclei.
The resulting droplets are extremely small and remain suspended in the atmosphere rather than immediately falling to the ground.
When enough droplets accumulate, visibility decreases and fog develops.
What Is the Dew Point?
The dew point is the temperature to which air must cool for it to become saturated with water vapour.
When the air temperature and dew point become very close, the relative humidity approaches saturation.
If additional cooling occurs, water vapour can begin condensing into liquid droplets.
This process frequently occurs near the ground overnight as the surface loses heat and cools the air immediately above it.
The relationship between temperature and dew point is therefore an important factor meteorologists consider when forecasting fog.
Why Does Fog Reduce Visibility?
Fog contains enormous numbers of microscopic water droplets suspended throughout the air.
These droplets scatter light travelling through the atmosphere.
As the concentration of droplets increases, less light from distant objects reaches an observer directly.
Objects consequently become increasingly difficult to distinguish.
The thicker the fog becomes, the shorter the distance at which objects can normally be seen.
Fog vs. Dense Fog
Fog is the general condition in which suspended droplets near the surface reduce visibility.
Dense fog describes particularly thick fog associated with much more substantial visibility reductions.
Dense fog can make roads, vehicles, signs and other objects difficult to see until they are relatively close.
The distinction is important because not every fog event produces the same degree of travel disruption.
Dense fog deserves its own condition page because it focuses specifically on severely restricted visibility and the resulting hazards.
Fog vs. Mist
Fog and mist are both composed primarily of tiny water droplets suspended near the surface.
The main practical difference is the degree to which visibility is reduced.
Fog produces a greater reduction in visibility, while mist generally describes less severe obscuration.
Both can occur when air near the surface becomes moist and approaches saturation.
The terminology and exact visibility distinctions can vary between meteorological and observational systems.
Fog vs. Freezing Fog
Freezing fog consists of supercooled liquid water droplets.
These droplets remain liquid even though their temperature is below freezing.
When they contact sufficiently cold surfaces, they can freeze and create deposits of ice.
Freezing fog therefore creates two potential hazards: reduced visibility and slippery or ice-coated surfaces.
Ordinary fog does not necessarily produce ice accumulation.
Fog vs. Ice Fog
Ice fog is different because it consists primarily of tiny ice crystals rather than liquid water droplets.
It develops under extremely cold conditions and is particularly relevant to northern and interior regions of Canada.
Both ice fog and ordinary fog reduce visibility, but the physical particles suspended in the air are different.
What Are the Different Types of Fog?
Fog can form through several atmospheric processes.
Common types include:
Radiation fog — develops as the ground cools overnight and cools moist air immediately above it.
Advection fog — develops when moist air moves across a colder surface and cools to saturation.
Upslope fog — forms as moist air moves uphill, expands and cools.
Steam fog — can form when cold air moves across relatively warmer water and receives moisture from evaporation.
Precipitation fog — can develop when precipitation adds moisture to cooler air near the surface.
These terms describe the process responsible for forming the fog rather than entirely different weather phenomena.
What Is Radiation Fog?
Radiation fog commonly develops during clear, relatively calm nights.
After sunset, the Earth’s surface loses heat through radiation.
The ground cools the air immediately above it.
If that air contains sufficient moisture, its temperature can fall toward the dew point.
Condensation begins and fog can form near the surface.
Radiation fog is particularly likely in low-lying areas because cooler, denser air can collect in valleys and depressions.
Why Does Fog Often Form in the Morning?
Nighttime provides favourable conditions for surface cooling.
Temperatures commonly decrease after sunset and reach their lowest levels near sunrise.
As the air temperature approaches the dew point, relative humidity increases.
Fog can therefore develop late at night or during the early morning hours.
After sunrise, solar heating warms the surface and surrounding air.
This can lower relative humidity and cause fog droplets to evaporate, allowing visibility to improve.
What Is Advection Fog?
Advection fog develops when moist air moves horizontally over a colder surface.
The colder surface cools the lowest portion of the air mass.
If the air cools to its dew point, condensation occurs and fog can form.
Unlike radiation fog, advection fog does not require calm conditions or nighttime cooling.
It can persist during the day and may affect relatively large areas.
Why Is Fog Common Near Water?
Lakes, oceans and other bodies of water provide abundant atmospheric moisture.
When moist air interacts with a colder surface or colder air mass, conditions can become favourable for fog.
Temperature differences between water and surrounding air are particularly important.
Canada’s extensive coastlines and enormous number of lakes create many environments where fog can develop.
Why Is Fog Common in Atlantic Canada?
Atlantic Canada is particularly well known for fog because different ocean temperatures and moisture-rich air frequently interact in the region.
When warm, humid air moves across colder ocean water, the lowest part of the air mass can cool toward saturation.
This can produce extensive marine and coastal fog.
Local winds can then transport fog inland into coastal communities.
Fog frequency varies considerably depending on season, location and ocean conditions.
Can Fog Form Around the Great Lakes?
Yes.
The Great Lakes can influence local moisture and temperature conditions throughout the year.
Fog can develop when moist air moves over colder water or when cold air interacts with relatively warmer lake surfaces.
Fog can also affect shorelines and nearby communities when winds transport moist air inland.
The exact formation mechanism depends on the season and temperature difference between the water and atmosphere.
What Is Valley Fog?
Valley fog forms when cool, moist air becomes concentrated in valleys and other low-lying terrain.
At night, air near hillsides and elevated terrain cools.
Because colder air is denser, it can drain downhill and collect at lower elevations.
If the accumulated air cools sufficiently to reach saturation, fog develops.
This means valleys can experience thick fog while nearby higher elevations remain relatively clear.
What Is Steam Fog?
Steam fog can develop when very cold air moves across relatively warm water.
Water evaporates from the warmer surface into the cold air above.
The added moisture quickly condenses, producing wisps or layers of fog that can appear to rise from the water.
This phenomenon is particularly noticeable over lakes and rivers during cold weather.
It is sometimes informally described as sea smoke when it occurs over marine environments.
Can Fog Form During Rain?
Yes.
Rain can contribute moisture to air near the ground.
When rain falls into cooler air, some of the water can evaporate.
This increases atmospheric moisture and can bring the air closer to saturation.
Fog associated with precipitation can occur near fronts and widespread rain systems.
Visibility may then be reduced by both rainfall and fog simultaneously.
Can Fog Form During Winter?
Yes.
Fog occurs during Canadian winters whenever suitable temperature and moisture conditions develop.
Winter fog may remain composed of supercooled liquid droplets even when temperatures are below 0°C.
This can produce freezing fog if those droplets freeze onto exposed surfaces.
Under much colder conditions, suspended ice crystals can instead produce ice fog.
Does Fog Freeze Below 0°C?
Not necessarily.
Tiny liquid water droplets can remain unfrozen at temperatures below 0°C. These are known as supercooled droplets.
If supercooled fog droplets contact a cold road, tree, aircraft or other surface, they can freeze on contact.
This is the process associated with freezing fog.
At extremely low temperatures, the composition of fog can shift toward suspended ice crystals.
How Does Wind Affect Fog?
Wind can either help or prevent fog formation depending on the situation.
Light winds can help mix moisture through a shallow layer near the ground while still allowing substantial cooling.
Completely calm conditions may allow condensation to remain extremely shallow near the surface.
Strong winds usually mix the lower atmosphere more vigorously, making some types of surface fog less likely.
However, advection fog relies on wind to transport moist air across a colder surface.
Wind therefore plays different roles depending on the fog-forming process.
How Long Does Fog Last?
Fog may last for less than an hour or persist for much of the day.
Radiation fog often begins dissipating after sunrise as temperatures rise.
Advection fog can persist considerably longer because the supply of cool, moist air may continue.
Cloud cover, wind, terrain, season and the strength of the temperature inversion can all influence how quickly fog clears.
In some coastal environments, fog can persist or repeatedly return while favourable atmospheric conditions remain in place.
What Causes Fog to Dissipate?
Fog dissipates when the conditions maintaining saturation change.
Several processes can cause this:
- Solar heating raises the air temperature
- Drier air moves into the area
- Winds increase atmospheric mixing
- The moisture source disappears
- A weather front changes the air mass
When the air temperature moves sufficiently above the dew point, relative humidity decreases and suspended droplets begin evaporating.
Visibility then improves.
What Is a Temperature Inversion?
A temperature inversion occurs when warmer air sits above cooler air near the surface.
Normally, temperatures decrease with increasing altitude through the lower atmosphere.
During an inversion, that pattern is reversed within a particular layer.
Inversions can trap cool, moist air near the ground and limit vertical mixing.
This can allow fog to persist longer than it otherwise would.
How Is Fog Forecast?
Meteorologists forecast fog by examining temperature, dew point, humidity, wind, cloud cover and atmospheric stability.
Weather models provide forecasts of near-surface temperature and moisture.
Forecasters also examine whether skies are expected to remain clear overnight and whether winds will become light enough for strong surface cooling.
Satellite observations can help identify existing areas of fog, particularly over large regions.
Surface weather stations provide direct measurements of visibility, temperature, humidity and other conditions.
Why Is Fog Difficult to Forecast?
Fog can be highly localized.
Small differences in temperature, wind or moisture can determine whether fog forms at one location but not another nearby.
Terrain creates additional complexity because cool air may collect in valleys while higher elevations remain clear.
Forecast models may correctly identify generally favourable conditions without precisely predicting where the thickest fog will develop.
Determining exactly when fog will dissipate can also be challenging.
How Is Visibility Measured?
Weather observations include estimates or measurements of horizontal visibility.
Visibility represents the distance at which objects or lights can be identified under prevailing atmospheric conditions.
Automated weather stations can use instruments that measure how strongly particles in the atmosphere scatter or reduce transmitted light.
These observations are especially important at airports, where visibility directly affects aviation operations.
How Does Fog Affect Driving?
Fog reduces the distance drivers can see ahead.
Road curves, intersections, pedestrians, stopped vehicles and other hazards may become visible with less time to react.
Dense patches can also cause visibility to change abruptly along the same road.
Drivers should reduce speed, increase following distance and use appropriate vehicle lighting when travelling through fog.
High beams can sometimes make visibility worse because their light reflects from suspended droplets back toward the driver.
How Does Fog Affect Aviation?
Fog can significantly affect airport operations because aircraft require sufficient visibility for many takeoff, landing and ground-movement procedures.
Dense fog may lead to delays, diversions or cancellations.
Airports use detailed visibility observations and specialized aviation forecasts to monitor changing conditions.
Modern aircraft and airports can operate under some low-visibility conditions, but allowable operations depend on equipment, procedures and the severity of the weather.
How Does Fog Affect Marine Travel?
Fog can create significant hazards on lakes, rivers and coastal waters.
Boats may have difficulty visually identifying shorelines, other vessels and navigational hazards.
Marine fog can also cover extensive areas and persist even when nearby land is relatively clear.
Navigation equipment, weather information and reduced operating speeds become particularly important when visibility deteriorates.
Fog in Canada
Fog can occur throughout Canada, but its frequency and causes vary considerably by region.
Coastal areas can experience fog when moist marine air interacts with colder ocean water.
Interior valleys can develop radiation fog during clear, calm conditions.
The Great Lakes influence fog formation across parts of Ontario and surrounding regions.
Northern Canada can experience conventional fog as well as ice fog during extremely cold conditions.
Because Canada experiences such a wide range of climates, fog can occur during every season and through several different formation mechanisms.
Fog vs. a Fog Advisory
Fog is the weather condition itself.
A Fog Advisory is an official weather alert issued when fog is expected to produce sufficiently poor visibility to create hazardous conditions.
Keeping these as separate pages is useful for Snowstorms.ca.
Your Fog condition page can explain the science, formation and impacts of fog, while your Fog Advisory page can focus on Canadian alert criteria, active advisories and what people should do when one is issued.
How to Stay Safe in Fog
When driving, reduce speed and leave additional distance between your vehicle and traffic ahead.
Use low-beam headlights or fog lights when appropriate rather than relying on high beams.
Avoid sudden braking or unnecessary lane changes when visibility is severely restricted.
People travelling by air or water should check current conditions because fog can affect flight and marine schedules even when other weather conditions appear relatively calm.
During winter, remember that fog occurring below freezing may also be associated with ice formation.